The present invention relates to a method of producing an aromatic polythiazole which is a rigid aromatic polymer having excellent heat resistance, mechanical properties, chemical resistance, electric properties, etc., particularly to a method of producing an aromatic polythiazole by using a usual organic solvent at a sufficiently low temperature.
Polymers having in their repeating units heterocyclic rings such as a thiazole ring, an imidazole ring, an oxazole ring, an oxazinone ring, etc. have been attracting much attention as high-strength, high-modulus, high-heat resistance polymers with high rigidity.
Among them, an aromatic polythiazole having a thiazole ring is expected to be used alone or in combination with other engineering plastics as a plastic material substitutable for metal materials.
However, since the aromatic diaminodithiol compound generally has a poor solubility in organic solvents, etc. due to its high rigidity, it is soluble only in limited kinds of strong acids such as metasulfonic acid, chlorosulfonic acid, etc. Accordingly, the molding of the aromatic diaminodithiol compound has been difficult.
The aromatic polythiazole is conventionally produced from an aromatic diaminodithiol compound and a dicarboxylic acid derivative, particularly its chloride, but once formed, its molding is generally difficult as mentioned above. Thus, it is general to produce an aromatic polythiazole prepolymer first and then to conduct its ring closure reaction to form an aromatic polythiazole. Since the aromatic polythiazole prepolymer is soluble in an organic solvent, by conducting the thiazole ring closure reaction after molding at the stage of a prepolymer, the difficulty of its molding can be avoided. As a result, formed products of the aromatic polythiazole having desired shapes can be obtained.
In the meantime, the prepolymer of an aromatic polythiazole has generally been produced by heating a mixture of an aromatic diaminodithiol compound or its salts and a dicarboxylic acid or its derivatives at a temperature of 160.degree. C. or higher in the presence of polyphosphoric acids, etc. However, in this case, it is difficult to control the polymerization reaction by heating in the presence of polyphosphoric acids. Thus, the polymerization reaction often proceeds to form a polythiazole. Therefore, various attempts have been made to provide the prepolymer of aromatic polythiazole.
Japanese Patent Laid-Open No. 60-223824 discloses a method of producing a polythiazole prepolymer by reacting an aromatic and/or heterocyclic diaminodithiol compound with a dicarboxylic acid in the presence of a carbodiimide.
However, since the above reaction is carried out in the presence of a carbodiimide, the actually formed polythiazole prepolymer has a low molecular weight only suitable for photoresists, despite the description that the reaction proceeds at such a temperature that the prepolymer is not converted to inactive ring-closed products. The above low-molecular weight prepolymer is not suitable as a prepolymer for high-strength polymers which are to be used in molecular composite materials.
In these circumstances, the applicant previously filed a patent application (Japanese Patent Application No. 63-136733) in Japan for a method of producing an aromatic polythiazole prepolymer, in which an aromatic diaminodithiol compound or its salt is polymerized with a dicarboxylic acid derivative by an interfacial polymerization method, and a patent application (Japanese Patent Application No. 63-140807) for a method of polymerizing an aromatic diaminodithiol compound salt with a dicarboxylic acid derivative in the presence of a phosphoric acid compound at a temperature of 70.degree.-140.degree. C.
The above methods can stably produce an aromatic polythiazole prepolymer more efficiently than any other conventional methods. However, in these methods, thiol groups of the aromatic diaminodithiol compound or its salt as a starting material are likely to react with the dicarboxylic acid derivative, and the thiol groups are partially subjected to a ring closure reaction in the process of forming the prepolymer, thereby producing insoluble components.